Yeah, that was the exact thing my google research did reveal. It's the "Don't add ';1'" option, the other one I had already enabled. The New .iso is even smaller now. I made two screenshots of the settings:
ISO9660 Settings
Allow Files Without Extensions and Don't Add ';1' Version Number To Files are the important Settings
Bootable Disc Settings
If you need a Boot Image, you can download one here (Note: german language and registration required).
I hope this is usefull to somebody else as well.